Fiber-optic cables can be installed in vertical and horizontal wells, which can be treatment wells, injector wells or observation wells. Within the cable there are often both single mode fibers for DAS and multi-mode fibers for DTS. Multiple fibers within one cable can offer redundancy and the ability to interrogate with different instrumentation simultaneously.
DAS is the measure of Rayleigh scatter distributed along the fiber optic cable. A coherent laser pulse is sent along the optic fiber, and scattering sites within the fiber cause the fiber to act as a distributed interferometer with a gauge length approximately equal to the pulse length. The intensity of the reflected light is measured as a function of time after transmission of the laser pulse. When the pulse has had time to travel the full length of the fiber and back, the next laser pulse can be sent along the fiber. Changes in the reflected intensity of successive pulses from the same region of fiber are caused by changes in the optical path length of that section of fiber. This type of system is very sensitive to both strain and temperature variations of the fiber and measurements can be made almost simultaneously at all sections of the fiber.
Raw DAS data are usually in the form of optical phase, with a range from −pi to +pi. The optical phase is defined by the interference pattern of the back-scattered laser energy at two locations separated by a certain length (gauge length) along the fiber. The phase varies linearly with a small length change between these two locations, which can be interpreted as axial strain change of the fiber in between. Depending on the vender, the measured optical phase is sometimes differentiated in time before it is stored. In this case, the DAS data can be considered as linear scaled fiber strain rates.
Prior to fiber optics, methods relied on acoustic stimuli (Godfrey, 2013), Doppler shifts (Godfrey and Crickmore, 2014), pressure pulses (Skinner et al., 2014) or spinners (Jaaskelainen et al., 2013),
DAS has been used to monitor hydraulic fracturing operation. The applications include injection fluid allocation (e.g. Broone et al. 2015), hydraulic fracture detection (e.g. Webster et al. 2013), and production allocation (e.g. Paleja et al. 2015). However, these applications focus on the DAS signals that are in high frequency bands (>1 Hz), and some applications only use the “intensity” of the signal (waterfall plot), which is obtained through a RMS averaging operation.
DAS has been used extensively to measure strain in hydrocarbon wells. Hill, et al., (U.S. Pat. No. 8,950,482) monitor hydraulic fracturing during oil/gas well formation. Tubel, et al., (US20060272809) control production operations using fiber optic devices. Hartog, et al., (US20090114386) use an optical fiber as a distributed interferometer that may be used to monitor the conduit, wellbore or reservoir. Minchau (US20130298665) provides an in-situ permanent method for measuring formation strain in a volume around a treatment well. McEwen-King (US20130233537) acoustic data from distributed acoustic sensing is processed together with flow properties data to provide an indication of at least one fracture characteristic. This is in no way an all-encompassing review of the technology. A recent review was published by Webster (2013) and the field has continued to advance rapidly.
Unfortunately, a common problem in optimizing the performance of horizontal wells stimulated via hydraulic fracturing is determining the relative amounts each fracture stage is contributing to the total oil production. Without this information, it is difficult to assess the effectiveness of various well treatment strategies during completion, or after production has commenced.